Pulse-width modulation (PWM) based switched-mode power supplies (SMPS) are common power supplies used in a variety of applications. PWM SMPS devices may be self-contained power supply units or may be elements of circuits such as vital circuits used in the railroad industry. In a PWM SMPS, DC Power fed to a load is controlled by opening and closing a switch between the supply and load rapidly to form pulses of transmitted power. These pulses are conditioned by capacitors and/or inductors into an approximately linear DC signal. In PWM-based power supplies, load current measurement may be performed to rapidly detect overload states or loss of capacitance, or for general load monitoring. This monitoring is useful because in a typical power supply components are not expected to change over the operational life of the system. Therefore, detected changes in current for a given load may indicate problems such as capacitor failure. Capacitor failure may cause an increase in the equivalent series resistance (ESR) of the device, which in turn can cause increased heating and physical electrolyte leakage. Reduced capacitance may also impair feedback loop response of the circuit. Load current is often determined by one of several sensing techniques that require sense connections to components in the circuit.
One technique for monitoring load current in the prior art uses either a high-side or low-side sense resistor on the load itself to determine output current. For example, in the circuit of FIG. 1, the PWM controller 100 controls the switch 140. When the switch 140 is closed, source 170 causes current to flow through transformer 150, which in turn supplies current to an inductor 160. The inductor 160 and capacitor 180 condition the current before it reaches the load 110. A resistor 120 is inserted between the inductor 160 and the load 110. A voltage drop across the resistor 120 is detected by a power supply sensor 130 which may include an analog to digital converter or an analog threshold detector. The voltage detected by the sensor 130 can be connected to a comparator or amplifier to facilitate current level sensing or overload monitoring. From this measured voltage drop and the known resistance of the resistor 120, load current can be determined. This method may be reasonably accurate, but the added resistor 120 adds to component cost, heat dissipation, and output voltage drop.
FIGS. 2A and 2B depict another prior art load current monitoring technique wherein a sense resistor on the switching element or the parasitic resistance of the switching element itself is used to find the current. As in FIG. 1, the PWM controller 200 controls the switch 240. When the switch 240 is closed, source 270 causes current to flow through transformer 250. The transformer 250, which in turn supplies current to an inductor 260. The inductor 260 and capacitor 280 condition the current before it reaches the load 210. A resistor may be inserted between switching element 240 and ground at 220 as in FIG. 2A, or the parasitic resistance 225 of the switching element 240 may be used as in FIG. 2B. In either case, sensor 230 measures the voltage drop across the resistor 220 or 225 to ground. The voltage detected by the sensor 130 can be connected to a comparator or amplifier to facilitate current level sensing or overload monitoring. From this measured voltage drop and the known resistance of the resistor 220 or 225, load current can be determined. As with FIG. 1, the addition of a resistor 220 in FIG. 2A adds to component cost, heat dissipation, and output voltage drop. The parasitic resistance 225 of FIG. 2B does not contribute to these problems (because the switch 240 must be present in any case), however the parasitic resistance 225 cannot be precisely known because of component lot variations and environmental impact on its value.
A prior art current sensing technique that does not use resistance to detect current is shown in FIG. 3. The PWM controller 300 supplies power from source 380 through the activation of switches 360 and 370. The inductor 320 and capacitor 390 condition the current before it reaches the load 310. Sensor 350 monitors node voltages at the node 330 shared by the load 310, inductor 320, and capacitor 390 and the node 340 shared by switches 360 and 370 and inductor 320. These node voltages are measured at certain points in time with respect to the actuation of the switches 360 and 370, and forward current in inductor 320 can be approximated from these measurements. In this technique, noise on the nodes 330 and 340 can reduce the accuracy of the current approximation.